The program objective is to understand the primary photochemical reactions involved in human photosensitivity mediated by drugs and metabolic products. Quantitative measurements will be made of the damage to liposomes induced by the attack of singlet oxygen, superoxide and hydroxyl radicals generated by reproducible physico-chemical methods. Related experiments of serum proteins will emphasize chemical damage and the alteration of receptor properties. Current work on primary photosensitizing mechanisms of psoralens will be continued using laser flash photolysis, including the effect of DNA binding on the competition between photoadduct formation and singlet oxygen generation.